Recently, there is an increasing tendency that the magnetic powder of finer grain size and higher magnetic property is used for the magnetic recording media especially for the video recording medium which is required to be capable of recording the short-wave signals. On the other hand, however, the smaller the grain size and the higher coercive force of the magnetic powder, the larger the cohesion of the individual grains, and this adversely affects the dispersibility and the surface smoothness of the recording medium which are necessary for adequate reproduction output of the recorded short-wave signals and the satisfactory S/N ratio. Furthermore, the recording medium using said kind of magnetic powder is vulnerable to the fall of the magnetic powder contained in the magnetic coating and the resultant clogging of the magnetic head which are detrimental to the function of the recording medium, since the repetitive use of such recording medium causes the wear of its magnetic coating.
Thus, some methods for improving the properties of the coating of the said magnetic powder have been proposed by now. Included in such methods are one featuring the improvement of the dispersibility of the magnetic powder in the binder through the adjustment of the grain size distribution of the magnetic powder, one featuring the use of the surface active agent as the dispersion medium and one featuring to change the properties of the magnetic coating through the introduction of the hydrophilic group such as the hydroxy group, phosphoric group, sulfonic group or carboxy group into the binder.
For example, there are methods such as one featuring the improvement of the surface smoothness (gloss) and the bonding strength to the backing material (of the magnetic coating) through the introduction of a metal sulfonate into the polyester resin disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I Publication) No. 74824/1971 and No. 74827/1971 and another one featuring the improvement of the dispersibility and the filling property associated with the interfacial tension (of the magnetic coating) disclosed in Japanese Patent O.P.I Publication No. 13519/1971, No. 74826/1971, No. 74828/1971 and No. 74829/1971. Besides, a number of proposals have been made concerning the compositions of the binder to be obtained through the combination of the polyester resin containing the metal sulfonate, polyurethane resin, nitrocellulose and vinyl chloride resin-vinyl acetate resin.
Furthermore, the resins of the vinyl chloride resins have relatively large mechanical strengthes, and so the copolymer of vinyl chloride-vinyl acetate, the compolymer of vinyl chloride-vilidene chloride, the copolymer of vinyl chloride-acrylonitrile or the copolymer of vinyl chloride-vinyl acetate and vinyl alcohol-maleic anhydride are mainly used.
The modification of properties of the vinyl chloride resins by the metal sulfonate group has been disclosed in Japanese Patent O.P.I. Publication No. 44227/1972. According to this disclosure, first the desalting reaction is made to occur between a resin of vinyl chloride family containing OH group and a compound which, for example, contains the chlorine atom and metal sulfonate in a molecule to obtain a resin of denatured vinyl chloride family into which sulfonic acid metal salt residual group is introduced.
A copolymer vinyl chloride family containing the free sulfo group (-SO.sub.3 H) is disclosed in Japanese Patent O.P.I. Publication No. 108032/1973.
Such a vinyl chloride copolymer obtained from the polymerized monomer containing the free sulfo group is sometimes advantageous as to the dispersibility of the magnetic powder but tends to increase its viscosity so rapidly before the cating following the dispersion that the coating becomes impossible in many instances.